In the manufacture of semiconductor devices and further products, ion implantation systems are used to impart dopant elements into semiconductor workpieces, display panels, glass substrates, and the like. Typical ion implantation systems or ion implanters implant a workpiece with an ion beam of impurities in order to produce n-type and/or p-type doped regions, or to form passivation layers in the workpiece. When used for doping semiconductors, the ion implantation system injects a selected ion species into the workpiece to produce the desired extrinsic material properties. Typically, dopant atoms or molecules are ionized and isolated, accelerated and/or decelerated, formed into a beam, and implanted into a workpiece. The dopant ions physically bombard and enter the surface of the workpiece, and typically come to rest below the workpiece surface in the crystalline lattice structure thereof.
A typical ion implantation system is generally a collection of sophisticated subsystems, wherein each subsystem performs a specific action on the dopant ions. Dopant elements can be introduced in gas form (e.g., a process gas) or in a solid form that is subsequently vaporized, wherein the dopant elements are positioned inside an ionization chamber and ionized by a suitable ionization process. Over the last decade the so-called “Bernas-style” ion source has become generally accepted as an industry standard for both high and medium current ion implantation systems. For example, the ionization chamber is maintained at a low pressure (e.g., a vacuum), wherein a filament, for example is located within the ionization chamber and heated to a point where electrons are emitted from the filament. Negatively-charged electrons from the filament are then attracted to an oppositely-charged anode within the chamber, wherein during the travel from the filament to the anode, the electrons collide with the dopant source elements (e.g., molecules or atoms), which results in the separation of electrons from the source gas material, thereby ionizing the source gas and creating a plasma, i.e., a plurality of positively charged ions and negatively charged electrons from the dopant source elements. The positively charged ions are subsequently “extracted” from the chamber through an extraction slit or aperture via an extraction electrode, wherein the ions are generally directed along an ion beam path toward the workpiece.
Heated filament cathodes of the type described above typically degrade rapidly over time. As a result, a common variation to this style of ion source has been developed and deployed in commercial ion implantation systems, which employs an Indirectly Heated Cathode (IHC), wherein the electron emitter is a cylindrical cathode, typically 10 mm in diameter and 5 mm thick, positioned within the ionization chamber. This cathode is heated by an electron beam extracted from a filament located behind the cathode, thereby protected from the harsh environment of the ionization chamber. An exemplary IHC ion source is shown, for example, in commonly assigned U.S. Pat. No. 5,497,006, among other patents.
In the case of a filament cathode, the cathode heater power is typically on the order of a few hundred watts, and in the case of an IHC, typically on the order of one kilowatt. When operating with standard implantation gases such as boron trifluoride (BF3), phosphine (PH3) and arsine (AsH3), typical maximum extracted ion beam currents are in the range of 50 to 100 mA, requiring a discharge power (cathode voltage times cathode current) of hundreds of watts. With these cathode heater powers and discharge powers, the walls of the ion source typically reach temperatures in excess of 400 degrees C. For operation with standard gases, these high wall temperatures are advantageous as condensation of phosphorus and arsenic on the walls is prevented, greatly reducing cross contamination when changing dopant species.
Substantial improvements in throughput have been demonstrated for low energy boron implants, for example using large molecules such as decaborane (B10H14) and octadecaborane (B18H22). Discharge powers and plasma densities in such large molecule plasmas must be maintained at much lower levels than for standard implant gases in order to prevent dissociation of the molecules. Typically, extracted ion currents are 5 to 10 mA requiring only tens of watts of discharge power. Though the standard sources described above can run stably at these low powers with standard implant gases, problems are encountered when running decaborane or octadecaborane. In the case of the Bernas source, where the filament is in contact with the gas, the filament is attacked by the borane and a stable discharge cannot be maintained. In the case of the IHC, the discharge is much more stable, but thermal dissociation of the large molecules is unacceptably high. Dissociation occurs both on the hot cathode and on the walls, which are difficult to maintain at low temperature due to the high radiative power of the cathode.
The problems described above, encountered when operating with gases such as decaborane and octadecaborane, can be overcome by removing the electron source from the ionization chamber. One such solution is described in U.S. Pat. No. 6,686,595, wherein a conventional broad beam electron gun is mounted external to the ionization chamber and the electron beam is guided through an aperture into the ionization chamber. However, in this source configuration electron current injected into the ionization chamber is limited to tens of milliamps due to fundamental limitations of electron gun design. Since operation with standard implant gases at the standard ion beam currents of 50 to 100 mA requires electron currents of hundreds of milliamps to amps, this ion source configuration is not suitable for such operation. Indeed, this problem has become well recognized by the ion implant system manufacturers, and at least one solution has been described, as for example in U.S. Pat. No. 7,022,999, wherein it has been proposed to configure the ionization chamber in two discrete modes of operation: one mode for low electron current ionization applications; and one mode for high electron current ionization applications. Alternatively, an ion source configuration has been proposed in U.S. Patent Application Publication No. US 2006/0169915, wherein first and second electron sources are located at opposite ends of and arc chamber, with each electron source being energized in one of a so-called “hot” operating mode and a “cold” operating mode.
Accordingly, a need exists for an ion source which can operate with low source wall temperature and low discharge power for large molecule gases (so-called “molecular species”) and with high wall temperature and high discharge power for standard implant gases (so-called “monomer species”) in order to meet more of the needs of the ion implantation industry.